Axially reciprocating displacers in power conversion machinery, such as Stirling cycle machines, incorporate clearance seals between portions of a reciprocating displacer and a housing. The displacer forms a movable piston within the housing. Movement of the displacer transfers working fluid back and forth between a compression space, having a high temperature space, and an expansion space, having a low temperature space. In one case, an end portion of a reciprocating displacer forms a drive area in fluid contact with the compression space. The displacer end portion slidably extends through a bore in the housing in fluid communication with a compression space of a linear drive motor. The drive motor has a driving piston that operates on working gas in the compression chamber. The working gas then directly works on the displacer to produce motion. Hence, the driving piston and displacer form a free piston machine, cooperation solely by action of the working fluid. A clearance seal is typically provided between the displacer end portion and the housing bore by maintaining an accurate reciprocating motion of the displacer and by providing an accurate relative sizing of the bore in the housing with the working piston and displacer end portion.
Techniques previously identified for achieving clearance seals in power conversion machinery include 1) precision machining of a bore with respect to a reciprocating displacer or piston member, 2) flexural bearings used to accurately position a reciprocating member with respect to a clearance seal, and 3) gas bearing supports/seals.
The present invention arose in part from an effort to improve the implementation of clearance seals in power conversion machinery. The general advantages of clearance seals include the following: improved efficiency resulting from elimination of sliding or rubbing surfaces in contact with one another; higher reliability resulting from elimination of contact wear that may produce detrimental seal leakage; extended life resulting from break-down of sliding or rubbing components leading to degradation of seals therebetween; lower cost resulting from elimination of separate sealing components that also add manufacturing steps during assembly; and less frictional wear and less generation of unwanted debris resulting from elimination of rubbing contact during start up and shut down; and reduced complexity by elimination of additional sealing components.
Coaxial non-rotating linear reciprocating members in power conversion machinery, such as Stirling Cycle Machines, incorporate gas flow paths to connect the compression space with the displacer assembly in the machine. The path allows working gas to be transferred back and forth between hot and cold working spaces; namely, the compression and expansion spaces. Particularly for small Stirling Cycle Coolers, a regenerator is placed in the flow path between the hot and cold spaces. In one version, the regenerator is carried by the moving displacer at a location remote from the compression space. A fluid flow path transfers working fluid between the compression space and expansion space, through the regenerator. The displacer reciprocates within a cylinder between the compression and expansion spaces, transferring working fluid in response to changes in the thermodynamic state of the working fluid. In response to the motion, the regenerator and flow path transfer fluid between the hot and cold ends provided by the compression and expansion spaces. A working gas is cooled as it flows through the regenerator, after which it is further cooled by expansion in the expansion chamber along a cold end of the displacer. Depending on the direction of fluid flow, the regenerator acts as a heat exchanger and a heat store. The resulting cooled working fluid is then able to better absorb heat at the cold end of the machine.
Small Stirling Cycle Coolers can be used for a number of cooling operations. One desirable application involves a cooler having a relatively small cold head configured to facilitate placement adjacent a sensor. For example, when detecting certain phenomena with a sensor, the sensor can be subjected to harsh thermal environments. Utilization of such a cooler enables placement of a sensor directly on the cold head of the cooler. Alternatively, a cryostat can be formed by placing the cold head in liquid nitrogen gas, in which the sensor is also positioned. Additionally, the cold head of such a cooler can be utilized to cool switching electronics, while maintaining relatively low noise from the device. Further alternative applications involve utilization of such a cooler to cool charge coupled devices, or even computer central processing units (CPU's).
One previously utilized technique for connecting a compression space with a regenerator and an expansion chamber involves providing a gas flow path by machining the path into the surrounding structure or body of the Stirling Cooler. Alternatively, a separate tube can be used to route the flow path between the compression and expansion chambers. Both of these approaches can complicate the mechanical complexity of the machine. Additionally, both approaches can result in high flow losses occurring along the flow path.
The present invention also arose from an effort to develop a thermodynamically improved flow path between the compression space and the displacer assembly. Improvements in maintaining a relatively small flow resistance while enhancing thermodynamic transference is just one of several benefits provided by the following implementations of the present invention. Another benefit provided by the following implementations is a clearance seal construction that is more easily and cost effectively implemented, greatly reducing any precision and accuracy requirements when machining and assembling components of the invention.